Speed Zones – Where Do They Change?

Q: When a road’s speed limit changes, let’s say from 35mph up to 50mph, does the increased speed limit take effect once you can see the new sign or only after you pass the sign?

A: I was curious about this myself, so I asked a traffic engineer the same question. He began by directing me to the Manual on Uniform Traffic Control Devices (MUTCD). This 800-plus page book is like the Bible to traffic engineers, providing guidance on all aspects of signs and markings on roadways. In the chapter on speed limits, the MUTCD states that speed limit signs “shall be located at the points of change from one speed limit to another.” Relying on the MUTCD, drivers should not accelerate until reaching an increased speed limit sign. I can already hear some grumbling about that answer. Keep reading. Beyond the MUTCD, the traffic engineer also brought up some additional points, including the reasons for speed limit changes and a variation on “speed kills.”

A speed limit can change just because the road spans jurisdictions. When going from a city into the county the speed limit may change to reflect the jurisdiction’s default speed limit. A speed limit will also increase or decrease due to road characteristics. Many factors determine the appropriate speed for a road; a few examples include road surface and width, frequency of curves, driver visibility, traffic density, urban vs. rural, pedestrian traffic, and number of driveways entering the road. You could say the first reason for the speed limit change is primarily political while the second is for safety. By the way, I don’t mean “political” in a negative way. It makes sense that the RCW specifies that the speed limit on any city road is 25 MPH unless otherwise posted and allows municipalities to adjust a few roads that are outliers, rather than do a speed study for every road and have speed limits ranging from 20 to 45 MPH in 5 MPH increments changing frequently throughout the city. As we’ve discussed before, predictability increases safety on roads.

Many of us are familiar with the expression, “speed kills”, and we know through experiences in our community that vehicles traveling at high speeds do, in fact, kill. However, we don’t often hear the term “speed differential kills.” You’ve probably noticed that most drivers travel at speeds close to the speed limit, varying by a few miles per hour, and it generally works pretty well. You may have even experienced times when drivers are all exceeding the speed limit by a similar amount and, while not legal, it doesn’t feel particularly dangerous. However, when a speeding driver encounters another driver traveling at or below the speed limit that’s a problem. Maybe seven to ten miles over the speed limit doesn’t seem like a big deal, but when that driver encounters someone driving at seven to ten miles under the speed limit, we now have a 14 to 20 mile per hour speed differential, shortening reaction times and increasing the risk of a crash.

Speed differential can also occur in a speed zone. If all the drivers around you speed up when the new speed limit sign is in view while you, the law abiding citizen, wait until reaching the sign to accelerate, by adhering to the letter of the law you create a speed differential. The inverse is also true, although probably less frequent; if all the drivers wait until reaching the speed limit sign to speed up and you accelerate as soon as you see the sign it creates that speed differential again.

Do you speed up and risk a ticket or hold your speed and risk a crash? I’m not going to answer that question for you, but I will propose some scenarios to consider. I’ve driven past a speed limit sign that reads “35 MPH unless otherwise posted” and within sight of that sign is a 45 MPH speed limit sign. I might infer that the first sign was to let me know that I should generally expect to drive 35 MPH on county roads, rather than an expectation that I drive 35 MPH until I reach the 45 MPH sign fifty yards later. In contrast, if I’m in a 20 MPH school zone and I can see a 35 MPH sign down the road, I’m not going to consider speeding up until I’m out of the school zone.

We’ve been discussing this primarily from a safety perspective, but what about enforcement? The goal of traffic enforcement is to reduce crashes, so a police department focused on traffic enforcement objectives will align its resources to enforce speeds in areas where crashes occur or where the consequences of a crash would be disproportionately tragic. In reviewing the two previous scenarios, we would expect to see speed enforcement in the school zone and surprised to see it in the fifty yards that changes from 35 to 45 MPH. As with any driving scenario, good judgment and a focus on safety will help drivers navigate speed limit changes. Be kind and drive wise.